Haven't seen or heard anything about fret wear with the new Cobalt strings. Just put a set on my bass, wow clarity is the word, but wonder about the frets.
I've had mine on my 25th for about a month and play 4 days a week (about 10 hrs). I just looked my frets over and didn't see any signs of wear. The Cobalts are smoother to the touch than regular Slinkys if that means anything to the fret wear.
If it's an issue, and I don't think it is, it'll be less of an issue that it would with the stainless steel strings, and I can't think of any complaints where they are concerned. The simple fact is that frets will wear with ANY string (unless of course you get your instrument refretted with stainless steel frets and they'll just wear strings out faster too)!